Whilst I can see the reasoning behind not being able to split the units, it does seem odd that an Attack is enough to build a bridge, but an assault isn't...

Ah, but it's not about the strength of the order...it's simply about the use of the card. My guess would be that Richard chose the Attack card because it balances card value, how common the card is, and how useful the card is.

For example:

If you had to use an Assault Card to place a bridge, players would only place a bridge in extreme cases because it would feel like a total waste of a valuable card. A Probe or Recon might not be valuable enough (players would gladly pay one of those to have a bridge) but the Attack card makes for a hard choice but not too hard.